We opened a new team account just yesterday, and today I've been busy inviting people into the system by their email address. They're all receiving the email ok, but the actual link that needs to be click is virtually invisible for some of them.
One of them said specifically: "I upgraded to MS Office 2013, and in Outlook, all of the links were not showing. Thankfully, in the web outlook version, they do."
Most of the email formats correctly, but the link goes to the bottom-left of the box it sits in, is missing its background and the text color is white.